#include <bits/stdc++.h>
using namespace std;
using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
using i128 = __int128;
const int N = 1e6 + 5;

void solve() {
    int n;
    cin >> n;
    vector<int> a(n);
    int sum = 0, ans = 0;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        sum += a[i];
    }
    if (sum != n * (n + 1) / 2) {
        cout << "-1" << endl;
        return;
    } else {
        sort(a.begin(), a.end());
        for (int i = 1; i <= n; i++) {
            ans += abs(a[i - 1] - i);
        }
        ans /= 2;
    }
    cout << ans << endl;

    return;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    solve();

    return 0;
}